Solar Energy Materials & Solar Cells 141 (2015) 171–177

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Solar Energy Materials & Solar Cells

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Charge transport model for photovoltaic devices based on printedpolymer: Fullerene nanoparticles

Natasha A.D. Yamamoto a,b, Margaret E. Payne a, Marlus Koehler b, Antonio Facchetti c,Lucimara S. Roman b, Ana C. Arias a,n

a Department of Electrical Engineering and Computer Science, University of California, Berkeley, CA 94720, United States of Americab Department of Physics, Federal University of Paraná, CP 19044, CEP 81531-980, Curitiba, PR, Brazilc Polyera Corporation, Skokie, IL 60077, United States of America

a r t i c l e i n f o

Article history:Received 19 March 2015Received in revised form22 May 2015Accepted 24 May 2015Available online 11 June 2015

Keywords:Electrical transport modelPolymer nanoparticleAqueous dispersionFlexible solar cellsDoctor blade coating

x.doi.org/10.1016/j.solmat.2015.05.03448/& 2015 Elsevier B.V. All rights reserved.

esponding author.ail address: acarias@eecs.berkeley.edu (A.C. Ar

a b s t r a c t

The electrical transport properties of films derived from aqueous semiconducting nanoparticle are fullydescribed by a phenomenological model that relates intrinsic film morphology to photovoltaic response.The model is applied to a new bulk heterojunction blend, composed of organic semiconducting nano-particles formed from the polymer donor poly[{2,6-(4,8-didodecylbenzo[1,2-b:4,5-b']dithiophene)}-alt-{5,5-(2,5-bis(2-butyloctyl)-3,6-dithiophen-2-yl-2,5dihydropyrrolo[3,4-c]57pyrrole-1,4-dione)}] (P(TBT-DPP)) and the fullerene indene-C60-bisadduct (ICBA) synthesized by the miniemulsion method. Thenanoparticle inks are printed from an aqueous dispersion onto flexible ITO-free substrates yieldingpower conversion efficiency of 2.6%.

& 2015 Elsevier B.V. All rights reserved.

1. Introduction

Solar cells based on organic semiconductors have attracted sig-nificant interest due to their potential for large area production,lightweight and new form factors [1,2]. These devices are oftenfabricated using printing [3–8] and coating techniques that arecompatible with flexible substrates enabling progress towards low-cost high speed production methods [9–11]. The optimization ofbulk heterojunction solar cells based on polymer:fullerene blendslead to recent reports of high performance, reaching up to 10%power conversion efficiencies (PCE) [12–14]. The high PCEsachieved with devices based on blends of electron-donating andelectron-accepting materials require fine control of thin film phaseseparation on the nanometer scale which leads to increased num-ber of interfaces where exciton dissociation and charge generationoccurs [15]. The resulting film morphology strongly depends onseveral processing parameters [16] including the solubility of thematerials in the solvent used [17], the interaction with the substratesurface [18] the packing of the molecules, formation of domains ofdifferent blend compositions [19] and the film deposition technique[20]. It has been established that exciton dissociation can be max-imized when donor and acceptor components are intimately mixed

ias).

and the length-scale of the phase separation is in the range of theexciton diffusion length [15,21]. As a result, over the years, manymethodologies to control the blend morphology have been devel-oped: appropriate solvent choice [22] controlling the rate of solventevaporation [23,24], thermal annealing [25,26] melting of bilayers[27] and adding solvent additives [28]. These different procedurespromote the formation of a phase separated morphology that leadsto improved photovoltaic performance. However the prediction andcontrol of the final film morphology is extremely challenging due tothe interplay between mixing and drying. Most laboratories usetoxic and flammable organic solvents in their fabrication process oforganic solar cells. Polymer nanoparticles dispersed in water are analternative method that addresses both morphology control andscalable on non-toxic ink deposition. The miniemulsion technique[29] retains the donor/acceptor phase segregation achieved at thesynthesis step regardless of the deposition solvent evaporation timeor film processing method. This technique was previously utilizedfor synthesis of blends of polyfluorenes [30,31] and poly(3-hex-ylthiophene)(P3HT): phenyl-C61-butyric acid methyl ester (PCBM)nanoparticles [32] deposited by blade coating achieving PCEs of0.29% [33] and 0.55% [34]. Solar cells based on spin coated blendnanoparticles of P3HT and the fullerene indene-C60-bisadduct(ICBA) resulted in power conversion efficiency of 2.50% [35].Although several photovoltaic devices based on aqueous nano-particle dispersions have been reported [30–32,35,36], the effects of

Fig. 1. (a) Simplified scheme of the nanoparticle synthesis: organic solvent solution containing P(TBT-DPP) and ICBA is added to an aqueous solution containing surfactant,this mixture is sonicated to form the nanoparticle dispersion and after a dialysis process the nanoparticle ink is ready for priting; (b) scheme describing the doctor bladecoating technique; (c) device structure: PEN/PEDOT:PSS/NP P(TBT-DPP):ICBA/C60/Al; (d) device photograph and (e) equivalent circuit used to model the dark J–V response ofthe devices: R is the series resistance, Rsh is the shunt resistance, Vsc is the potential drop across R, JD is the diode current, Jsh is the shunt current and J is the total current ofthe system.

Table 1Composition and properties of the P(TBT-DPP):ICBA nanoparticles.

Blend weightratio

Mass of P(TBT-DPP)[mg]

Mass ofICBA [mg]

Mean diameter[nm]

η ρ [nm]-3

(1:1) 20 20 80 1.18 4.40�10�6

(3:7) 12 28 85 2.75 8.55�10�6

(1:3) 10 30 110 3.54 5.08�10�6

N.A.D. Yamamoto et al. / Solar Energy Materials & Solar Cells 141 (2015) 171–177172

this type of donor/acceptor phase segregation on the electricalproperties remains poorly explored. We fabricated blade coateddevices using conventional chloroform bulk heterojunction (BHJ)and nanoparticle dispersed in aqueous media (NP) to form theactive layer of flexible solar cells. We characterized the film mor-phology and electrical properties and fitted the experimental darkJ–V curves using a phenomenological electric model. The nano-particle composition and morphology were correlated to thephotovoltaic performance and we demonstrate that the chargetransport in nanoparticulate devices is improved when compared tothe BHJ. The photovoltaic active layer of the flexible solar cellsreported here, is composed of a new nanoparticulate bulk hetero-junction blend of the polymer poly[{2,6-(4,8-didodecylbenzo[1,2-b:4,5-b']dithiophene)}-alt-{5,5-(2,5-bis(2-butyloctyl)-3,6-dithio-phen-2-yl-2,5dihydropyrrolo[3]57pyrrole-1,4-dione)}] (P(TBT-DPP))[37] with ICBA. The P(TBT-DPP):ICBA nanoparticles were synthe-sized in aqueous media following the miniemulsion techniqueillustrated in Fig. 1a. The aqueous nanoparticle (NP) inks wereprinted by doctor blade coating in air [38] onto PEN/PEDOT:PSSsubstrates as depicted in Fig. 1b and a schematic representation ofthe device structure is shown in Fig. 1c. Conventional bulk hetero-junction (BHJ) devices were also deposited by blade coating andused as control devices. Our model based on [39,40] can be appliedto fit the dark J–V curves of both conventional BHJ and nanoparti-culate devices. Our model assumes that the dark J–V curves can beobtained using the equivalent circuit illustrated in Fig. 1e. Themodel is composed of a nonlinear space-charge limited seriesresistance (R), that depends on J, connected to a diode (D) in parallelto a shunt resistance (Rsh) [39,40]. By means of this Space-ChargeLimited-Diode (SCL-D) model, relevant parameters such as effectivemobility (meff) and the diode saturation current (Jo) are quantifiedand related to the photovoltaic response of the devices.

2. Experimental section

Synthesis and characterization of the nanoparticle inks: disper-sions were prepared using P(TBT-DPP) provided by Polyera andICBA (purchased from Sigma Aldrich). Chloroform solutions of P(TBT-DPP) and ICBA (40 mg/mL) in three donor:acceptor weightratios (DAWR) (1:1, 3:7 and 1:3) were added to a 5 mg/mL aqueoussodium dodecyl sulfate (SDS) solution. Preemulsification isachieved by stirring the resulting solution at 1000 rpm during 1 h.

Subsequently the mixture was sonicated using a Branson 450digital sonifier for 2 min at 60% amplitude with a microtip (radius6.5 mm) immersed in the emulsion to form the dispersion. Thenthe samples were heated while stirring for 30 min at 65 °C toevaporate the chloroform forming a stable aqueous dispersion. Inorder to concentrate and eliminate the excess surfactant, dialysisof the nanoparticle dispersion was performed using ultra cen-trifuge dialysis tubes with 10 kDa molecular weight cutoff (pur-chased from Millipore). The dialysis procedure was repeated untilthe surface tension of the filtrated water reached about 38 mN/m.Dynamic light scattering (Zetasizer Nano-ZS, Malvern Instru-ments) was used to measure the size distribution of the nano-particles in the aqueous dispersion. The nanoparticle compositionand average particle size measured by Dynamic Light Scatteringare summarized in Table 1. The nanoparticle size strongly dependson the initial ratio of surfactant, polymer, fullerene, organic solventand water. Therefore the synthesis parameters can be tuned inorder to control the final particle size. For a given blend ratio,higher amount of blend material leads to larger particle size [41]whereas the increase in the surfactant amount reduces the particlediameter [29]. In this work, the total blend concentration was keptconstant at 40 mg/mL and it is shown that the average diameterincreases with the amount of ICBA in the blend. The molecularweight of the polymer is 27 kDa [37]. Since the monomer mole-cular weight of the P(TBT-DPP) is approximately 1.12 kDa, theweight-average degree of polymerization ( nw ) is �24. Themolecular weight of ICBA, the acceptor, is 952.96 Da [42]. Based onthe molecular weight values for donor and acceptor materials wecan calculate the ratio (η) of ICBA molecules per polymer chain foreach nanoparticle blend composition. The average density of ICBAmolecules per monomer of the polymer inside the nanoparticle (ρ)is then given by ρ¼η /V, where V is the average volume of thenanoparticle (assuming a spherical structure with the diameter

N.A.D. Yamamoto et al. / Solar Energy Materials & Solar Cells 141 (2015) 171–177 173

given by the DLS measurements). As shown in Table 1, the nano-particle diameter (and its volume) increases as η increases (higheracceptor concentration). However, the density of ICBA per mono-mer (ρ) reaches a maximum for the 3:7 samples. Optimizednanoparticles inks were prepared with the addition of 20% ofethanol by volume to the NP dispersion in the DAWR of 3:7 afterthe dialysis procedure.

Solar cells fabrication and characterization: a double layer ofdifferent poly(ethylene dioxythiophene) doped with poly(styr-enesulfonic acid) (PEDOT:PSS) compositions is used instead of ITOas the transparent electrode. The first layer of 200 nm thick pro-vides higher conductivity using Sigma Aldrich 739316-25G whichhas work function of 5.15 eV. This layer was deposited onto pre-cleaned polyethylenenaphthalate (PEN) (DuPont PQA1) substratesusing a height-adjustable doctor blade (Zehntner ZUA 2000.60) setup in ambient conditions with a linear servo (Servo City). Theblade distance from the substrate was set to be 350 mm. ThePEDOT:PSS film was dried in a hot plate at 120 °C for 25 min.Subsequently the second PEDOT:PSS layer (CleviosTM AI 4083) wasdeposited to modify the electrode work function to 5.30 eV. Thesecond layer is 100 nm thick and after deposition the film is driedat 120 °C for 25 min. The dispersions of the P(TBT-DPP):ICBAnanoparticles were doctor blade coated in air onto the PEN/PEDOT:PSS substrates and annealed at 150 °C for 20 min. Nano-particle film thicknesses ranged from 100 to 150 nm. The deviceswere completed with the deposition of C60 (40 nm) and Al(100 nm) layers thermally evaporated through a shadow mask atpressure of 5�10–6 mbar. The devices active area was found to be4 mm2. Conventional bulk heterojunction (BHJ) made fromchloroform solutions with the same blend ratios were bladecoated onto PEN/PEDOT:PSS substrates in the exact same condi-tions as the nanoparticles (NP) devices and used as control devicesin our studies. For the optimized NP devices, the PEN substrateswere exposed to (heptadecafluoro-1,1,2,2-tetrahydrodecyl)tri-chlorosilane (FDTS, Gelest SIH5841.0) for 15 min under lightvacuum (0.1–1 Torr) [38]. A mask defining the area where thePEDOT:PSS and NP layers will be coated is pressed flush againstthe substrates then oxygen plasma-treated for 15 s at 50% power.The ethanol modified dispersion of the P(TBT-DPP):ICBA (3:7)nanoparticles were doctor blade coated in air onto the PEN/PEDOT:PSS substrates and the devices were completed with theC60 (40 nm) and Al (100 nm) deposition. Thickness measurementswere performed with a Dektak 150 profilometer (Veeco Instru-ments) and film morphology was investigated using an AsylumMFP-3D Atomic Force Microscope operating in dynamic mode.Photovoltaic characterization was performed with a Keithleypicoammeter with power supply. The solar illumination conditionwas simulated with air mass (AM 1.5) filter and power illumina-tion of 100 mW/cm2 from a Xenon lamp.

3. Results and discussion

From Fig. 1(e), the current of the system is given by J J Jsh D= +where Jsh is the shunt current density and JD is the diode currentdensity given by the expressions J V V R A/sh sc sh= ( − ) ( ) andJ J q V V kTexp / 1D sc0= ( [ ( − ) ( )] − ) respectively. A is the device activearea, J0 is the diode saturation current and Vsc is the potential dropacross a space charge limited series resistance (R), given by

V JL1/3 8 /SC eff3 μ ε= ( ) [39], where L is the film thickness and ε is the

dielectric constant of the active layer ( 3 0ε ε∼ ) [43]. This value ofpermittivity is commonly assumed for the evaluation of chargecarrier mobility of polymeric semiconductors using space-chargelimited currents. Therefore the current of the system can bewritten as

J J J q V V kTexp / 1 1sh sc0= + ( [ ( − ) ( )] − ) ( )

The system theoretical dark J–V characteristics can be obtainedby solving Eq. (1) for a given J, where J0 and Rsh are fitting para-meters. From the Mott–Gurney law, Rsh this is given by

R L A V V8 /9 2sh efflow

sc3 εμ= ( − ) ( )

where L is the sample thickness and efflowμ is the fitting para-

meter that represents the effective mobility at low V.Applying the SCL-D model to the dark experimental J–V curves

gives the charge carriers effective mobility (μeff) which is also afitting parameter that determines the space-charge limited (SCL)current at high voltages. The shunt resistance, Rsh, controls theohmic behavior at low voltages and J0 is given by the steepincrease in the current between the ohmic and the SCL regimes[39]. Both, J0 and μeff depend on the intrinsic morphology of thephotoactive layer and can be directly related to the photovoltaicresponse of the device. For instance, small J0 is associated withhigher open circuit voltages by the expressionV kT e J J/ ln /oc

calcsc 0( )= ( ) [44]. Moreover the diode current can be

written as [40]

J J E kTexp /2 3mat DA0 ( )= − ( )

where EDA is the effective energy barrier at the donor/acceptorinterface and Jmat is an independent factor determining the carrierrecombination rate [40]. Jmat quantifies the inter-molecular overlapof the donor and acceptor molecules and the effective area of thedonor/acceptor interface. By the same token, μeff is also stronglyaffected by the morphology of the film at the molecular level.Large domains of donor (acceptor) aggregates create large barriersfor hole (electron) transport. The increased volume of those phasestends then to decrease the number of percolation paths that linkthe dissociation centers to the metallic contacts. As a consequence,large domains of polymer (fullerene) would tend to block theelectron (hole) transport toward the collecting electrodes. Thiseffect increases the chances of recombination during free carrierdrift that follows exciton dissociation. A higher rate of recombi-nation decreases μeff which results in lower short circuit currents.The film morphologies for compositions of 3:7 (DAWR) forchloroform BHJ and aqueous NP devices were accessed usingatomic force microscopy (AFM) and the images are displayed inFig. 2a and b, respectively and the phase contrast image simulta-neously acquired are shown in Fig. 2c and d. The conventional BHJfilm blade coated from chloroform solutions resulted in largedomains in the order of 500 nm of diameter. In contrast, the P(TBT-DPP):ICBA nanoparticle film exhibited finer phase segrega-tion on the order of 100 nm, which is consistent with the DLSmeasurement for the aqueous dispersion. These images confirmthat blend phase segregation fixed during the nanoparticlesynthesis is maintained during film deposition and solvent drying.Fig. 2e and f shows that there is a good agreement between themodel proposed here and the experimental data obtained for bothtypes of morphology. In Table 2 we report the values of μeff, J0 andRsh obtained from the model at V¼ 0.1 V. As displayed in Table 2,μeff and J0 increase for the devices made with higher ICBA densitynanoparticle solutions. The larger domains of donor (acceptor)shown in the BHJ film produce higher fluctuations in the localdensity of ICBA molecules. Those fluctuations mask the depen-dence of μeff and J0 on the bulk density of ICBA that is onlyapparent in the NP devices where the mixing of the acceptor anddonor materials is finer and more evenly distributed throughoutthe film. The values in Table 2 clearly show that the NP devicesalways give higher μeff and J0 when compared to the respective BHJcells for the same DAWR.

Fig. 2. Atomic Force Microscopy (AFM) topography map of (a) conventional P(TBT-DPP):ICBA BHJ and (b) NP films blade coated onto PEN/PEDOT:PSS substrates and phasecontrast images simultaneously acquired for (c) conventional P(TBT-DPP):ICBA BHJ and (d) NP films. The scan size is 2 mm x 2 mm. J–V curves in the dark from conventional(e) BHJ and (f) NP devices based on the blend P(TBT-DPP):ICBA in the weight ratios: 1:1 (triangle), 3:7 (square) and 1:3 (circles). Solid lines are obtained from the fitting ofthe experimental curves using the proposed model.

Table 2Fitting parameters of the experimental dark J–V curves from Fig. 2 taking shuntresistance values given by Eq. (2) at V¼0.1 V.

Blendweightratio

J0 [mA cm-2] μeff [cm2 V-1 s-1] Rsh [Ω cm2] Vocexp [V] Voc

calc [V]

BHJ (1:1) 3�10�8 6.5�10�14 1.43�105 0.5270.09 0.45NP (1:1) 4�10�8 7.0�10�12 7.90�105 0.4570.02 0.47BHJ (3:7) 1�10�9 5.0�10�12 5.66�105 0.5570.01 0.56NP (3:7) 2�10�7 1.0�10�11 3.34�105 0.4270.01 0.42BHJ (1:3) 1�10�9 1.0�10�12 1.66�105 0.5770.01 0.55NP (1:3) 2�10�8 8.0�10�12 1.43�104 0.4970.03 0.48

N.A.D. Yamamoto et al. / Solar Energy Materials & Solar Cells 141 (2015) 171–177174

The J–V curves under illumination for conventional BHJ and NPdevices in the blend DAWR of 1:1, 3:7 and 1:3 are shown in Fig. 3and all the photovoltaic parameters obtained from these curvesare summarized in Table 3.

It was observed in similar polymer/acceptor nanoparticulatephotovoltaic devices that NP materials show evidence of a higherinter-chain order compared to the BHJ films [32]. This order tendsto increase the effective mobility for the NP devices, increasing Jmat

given in Eq. (3) which enhances J0. In contrast, the lower level ofcontrol over the nanoscale morphology in BHJ devices wouldreduce the effective donor/acceptor heterojunction area, leading toa lower Jmat. As a consequence, the NP devices have higher J0 whencompared to the BHJ devices for the same DAWR. Higher J0translates into lower values of Voc [44]. The NP devices generatelower open circuit voltages under illumination (100 mW cm–2)when compared to the respective BHJ devices as it is shown in

Table 2. This behavior was also reported when other NP systemswere compared to the corresponding BHJ [32,36]. The higherdensity ρ of ICBA molecules per monomer of polymer in the NP

Fig. 3. J–V curves under AM 1.5 illumination of 100 mW/cm2 from conventional BHJ and NP devices based on the blend P(TBT-DPP):ICBA in the weight ratios: (a) 1:1, (b) 3:7and (c) 1:3.

Table 3Device characteristics of the conventional BHJ and NP tested under AM 1.5 conditions (from Fig. 3).

Blend weight ratio Active layer thickness [nm] Jsc [mA cm�2] Voc [V] FF [%] Rs [Ω cm2] PCE [%]

BHJ (1:1) 140710 1.1770.21 0.5270.09 3076 285772 0.1870.03NP (1:1) 130710 2.1470.24 0.4570.02 4576 47711 0.4470.05BHJ (3:7) 12078 2.1270.08 0.5570.01 3773 83710 0.4470.02NP (3:7) 110715 2.5370.13 0.4270.01 5275 3177 0.5670.05BHJ (1:3) 15075 1.8070.27 0.5770.01 4473 71717 0.4470.06NP (1:3) 140712 2.3770.18 0.4970.03 4475 64713 0.5270.08NPa(3:7) 100726 10.4972.65 0.4370.01 4775 1273 2.1670.50NPb(3:7) 100726 12.73 0.44 47 11 2.63

a Optimized NP devices with the addition of 20% of ethanol in the NP ink.b Best performing device (NP 3:7 with 20% EtOH), values taken from Fig. 5.

N.A.D. Yamamoto et al. / Solar Energy Materials & Solar Cells 141 (2015) 171–177 175

sample may increase the electronic coupling between donor andacceptor and the effective area of the resulting heterojunctionenhancing Jmat. Additionally J0 is maximum for the NP sample withthe 3:7 DAWR (see Table 2). Consequently it is not surprising thatthe 3:7 device showed the lower Voc among all tested solar cells(see Table 2). In fact using the relation V kT e J J/ ln /oc

calcsc 0( )= ( ) , the

calculated values of the open circuit voltage match the measuredVoc values closely except for the BHJ device with DAWR of 1:1. TheJ–V curve of the BHJ 1:1 device does not describe an ohmicbehavior at very low applied voltages. Since our model assumesonly a transport-limited electrical conduction, the fit of this theoryto BHJ 1:1 experimental data overestimates the dark current (J0) atlow voltages. As a consequence, the calculated Voc underestimatethe experimental Voc.

A core–shell structure formed of an acceptor rich core sur-rounded by a polymer-rich shell was previously observed in othernanoparticulate donor/acceptor films [32,37]. Since in thosestructures the acceptor is reasonably dispersed inside the poly-meric shell, the transport would be essentially limited by thehopping of the electrons across thin polymeric layers coating theadjacent ICBA molecules. Assuming that the polymer acts as apotential barrier to inter-acceptor electron hopping, the transportof electrons between adjacent electronic states localized in theacceptor at constant temperature may depend only on the tun-neling part of the hopping rate. It is then reasonable to assumethat deffμ ∝ − [45], where d is the average distance betweenneighbor ICBA molecules in the NP. Due to spatial considerationsand neglecting the volume of the fullerene molecule compared tothe volume of the NP's, one can write d V4 /3 /2 3η π( )( ) ∝ . It followsthen that d 1/3ρ∝ − . This suggests that the relation eff

1/3μ ρ∝ − − isvalid for the NP structures used in our study. We use this relation

and plot in Fig. 4 the effective mobilities from Table 2 as a functionof the respective values of ρ taken from Table 1. The predictedlinear behavior is observed, with the solid line as a fit to the μeff

data. This result indicates that the transport in the NP samples arelimited by the inter-acceptor hopping of electrons across the bar-riers formed by the polymeric coating due to the finer donor/acceptor mix in the NP films.

Following the analysis above, the reasons behind the bestphotovoltaic performance obtained for the 3:7 NP solar cell arebetter understood. First the substantial decrease of the seriesresistance exponentially increases the effective mobility of the 3:7NP solar cell. Additionally, the higher concentration of donor/acceptor heterojunctions in this sample (higher ρ) increases thedensity of photoexcited excitons upon illumination which enhan-ces Jsc. Both phenomena compensate the lower Voc resulting in thehighest PCE compared to the conventional BHJ devices we haveanalyzed.

Given the good performance shown by the devices with DAWRof 3:7, we further optimized the ink formulation to improvewettability and promote better film formation. 20% of ethanol involume was added to the initial 3:7 to decrease the surface tension[46] of the nanoparticle aqueous dispersions. The addition ofethanol to the nanoparticle ink reduces the contact angle betweenink and substrate, PEN/PEDOT:PSS, from 59° to 45° leading to amore homogeneous film coating. Fig. 5 displays the J–V curveunder illumination of 100 mW cm–2 for the optimized NP deviceand the conventional BHJ in the same DAWR and blade coated inthe same conditions. The NP device exhibited six fold increase inthe Jsc (2.12 mA cm–2 for BHJ and 12.73 mA cm–2 for the NP)enhancing the PCE from 0.44% to 2.63%. This result is comparableto the highest PCE reported for P3HT:ICBA nanoparticles dispersedin water and spin coated onto ITO/PEDOT:PSS substrates (2.50%)

Fig. 4. Logarithm of the effective mobility as a function of ρ–1/3. The solid line is alinear fit to the data.

Fig. 5. J–V curves under AM 1.5 illumination of 100 mW/cm2 from conventionalBHJ and optimized NP (addition of 20% of EtOH) devices based on the blend P(TBT-DPP):ICBA in the weight ratio of 3:7.

N.A.D. Yamamoto et al. / Solar Energy Materials & Solar Cells 141 (2015) 171–177176

[35]. As it is shown in Fig. 5, despite the decrease in the Voc, 0.55–0.44 V, the NP device has shown remarkable improvement in thefill factor (FF): from 37% (BHJ) to 46% (NP). This result is consistentwith the decrease in the series resistance (Rs): from 83Ω cm2

(BHJ) to 11Ω cm2 (NP). We attribute the improved photovoltaicresponse found for the NP devices to the nanostructured filmmorphology.

4. Conclusions

We have fully described the transport properties of P(TBT-DPP):ICBA nanoparticles devices applying the SCL-D model to fitthe dark J–V curves and demonstrate that the charge transport isimproved for the NP when compared to conventional BHJ devicesdue to the closer contact between donor/acceptor in the nano-particle structure. We synthesized P(TBT-DPP):ICBA nanoparticlesin aqueous dispersion in three donor to acceptor weight ratios andall the NP devices have shown improved performance whencompared to the corresponding BHJ blends coated using the samelaboratory conditions. The NP devices exhibited higher short cur-rent density, higher fill factor and lower series resistance. More-over the effective mobility dependency with the density of ICBAmolecules per monomer of the polymer inside the NP indicatesthat the transport in the NP film is limited by the inter-acceptorhopping of electrons across barriers formed by the polymericcoating due to the finer donor/acceptor mix in the film. The bestblend ratio was found to be (3:7) with optimized devices yieldingpower conversion efficiency of 2.63%. The aqueous nanoparticleink significantly reduces the use of hazardous solvents in organicelectronics. In addition, it allows control of the donor/acceptormorphology at the nanometer scale without compromising

reproducibility in large area devices, such as photovoltaics. Ourresults confirm the viability of applying aqueous inks based onpolymer:fullerene nanoparticles for large-area photovoltaics thatare compatible with ITO-free flexible substrates and printingtechniques.

Acknowledgments

This work is based uponwork supported in part by the NationalScience Foundation under Cooperative Agreement no. EEC-1160494. Any opinions, findings and conclusions or recommen-dations expressed in this material are those of the author(s) anddo not necessarily reflect the views of the National ScienceFoundation. The authors would also like to thank Polyera forproviding materials and CNPq and CAPES–PDSE (process BEX –

1564129) for partial financial support.

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